To write the one-variable, one-step equation based on the information provided:
Let \( q \) represent the number of quarters.
Since Matthew has twice as many quarters as dimes, and we know he has 20 dimes, we can express the relationship as:
\( q = 2 \times \text{(number of dimes)} \)
Substituting the number of dimes (20) into the equation:
\( q = 2 \times 20 \)
Thus, the equation can be filled in as:
\[ q = 40 \]
So the completed statement would be:
\[ q = 40 \]